This invention relates to active target detection and location systems such as radar, and more particularly to the resolution of plural, closely spaced targets.
radar has long been used for area surveillance and tracking of targets. Early radar systems had trouble resolving plural targets which were at approximately the same bearing and at similar ranges, in part because the angular resolution of the radar system was limited by the beamwidth of the radar antenna. As a general rule, a radar antenna can distinguish two targets from one if their angular separation exceeds approximately 0.8 times the 3 dB receive beamwidth. This suggests that improvement in resolving closely spaced targets can be achieved by the simple expedient of decreasing the 3 dB beamwidth of the radar receive antenna. However, the beamwidth of an antenna cannot be decreased without limit, as reduced beamwidth requires a larger antenna dimension, and many radar uses place limits on those dimensions. Additionally, reduced 3 dB beamwidth requires that a scanning antenna move more slowly than an antenna with a greater beamwidth, if surveillance coverage is to be reliable. Monopulse techniques were introduced to aid in resolving closely spaced targets with antennas of reasonable dimensions.
Monopulse radar systems use antenna arrangements which generate slightly diverging plural beams in a given direction. For transmission of a radar pulse or burst, the sum of the plural beams is used. Monopulse reception involves processing which takes the difference between the signals received on mutually adjacent beams, thereby forming or defining phase and amplitude responses within the sum beam which can be exploited to determine the angle of the target relative to the boresight of the antenna. However, monopulse techniques do not, in general, improve the resolution between closely spaced targets.
Improved resolution between closely spaced targets is desired in a monopulse context.
A monopulse radar method according to an aspect of the invention is for detecting closely spaced targets. The method includes the steps of transmitting a first burst of electromagnetic pulses at a first frequency toward a region which may contain a target, and, using monopulse techniques, estimating azimuth and elevation angles of the target from return signals arising from the first burst and in a particular range cell or bin. The method includes the further steps of transmitting toward the region a second burst of electromagnetic pulses at a second frequency, different from the first frequency, and, using monopulse techniques, estimating azimuth and elevation angles from return signals in the particular range cell and arising from the second burst. A third burst of electromagnetic pulses is transmitted toward the region at a third frequency, different from the first and second frequencies, and using monopulse techniques, the azimuth and elevation angles are estimated from return signals arising from the third burst and in the particular range cell. An arithmetic value associated with the mean value of the first, second, and third azimuth and elevation estimates is computed to form a test statistic, the magnitude of which is indicative of the number of targets present in the range cell. The test statistic is compared with a decision threshold, and but a single target is deemed to be present in the range cell if the test statistic is less than the decision threshold, and two or plural targets are deemed to be present in the range cell if the test statistic is greater than the decision threshold.
A monopulse radar system according to an aspect of the invention comprises a transmitter arrangement for recurrently transmitting one of radar pulses and bursts, the frequencies of which are controllable. In this context, a burst includes a plurality of pulses. A monopulse receiving system includes a mechanism for generating an elevation difference beam and an azimuth difference beam responsive to target echoes originating from the one of radar pulses and bursts. A monopulse processor generates monopulse estimates of the azimuth and elevation angle of the centroid of clustered scatterers making up target(s). The system also includes means coupled to the transmitting means and to the monopulse processing means for causing the transmitter arrangement to transmit plural ones of the one of radar pulses and bursts, and for generating a test statistic representative of the shape of the cluster of the monopulse estimates arising from the plural ones. A comparator compares the test statistic with a decision value to declare the presence of a single target when the test statistic is one of greater than or less than the decision value and to declare the presence of plural targets when the test statistic is the other one of greater than or less than the decision value.